Rankine system with gravity-driven pump

ABSTRACT

A gravity-driven pumping unit has an inlet valve connected to a condenser, an outlet valve connected to a boiler, and a staging zone between the inlet and outlet valves. The inlet valve, the outlet valve, the liquid line and entire path established between the condenser and boiler are oriented, sized and shaped to allow for the vapor refrigerant to freely move upward from the boiler to the condenser and to allow for the liquid refrigerant to freely drain downwards from the condenser to the boiler by gravity. A control system opens and closes the inlet and outlet valves in a proper sequence, which enables gravity-driven movement of liquid refrigerant from the condenser to the staging zone and then from the staging zone to the boiler, against a positive pressure differential between the boiler and condenser.

FIELD OF THE INVENTION

The present invention relates to combined heat and power systems operating on the Rankine cycle that may or may not incorporate cogeneration. More particularly, the present invention relates to a pumping method and apparatus therefor.

BACKGROUND OF THE INVENTION

The Rankine cycle comprising a closed refrigerant loop, a condenser unit, a liquid refrigerant pump, a boiler unit, and an expansion machine is well known in the art. The condenser unit provides thermal contact and heat transfer interaction between a fluid to be heated and refrigerant to be condensed. The boiler unit provides thermal contact and heat transfer interaction between a fluid carrying enthalpy of available thermal energy and refrigerant vapor to be boiled. Such a system is described, for instance, in U.S. Pat. No. 3,393,515.

The liquid refrigerant pump recycles the condensed refrigerant to the boiler unit, substantially elevating pressure from a condensing pressure to a boiling pressure. Performing this function, the liquid refrigerant pump needs substantially subcooled liquid at the pump inlet to avoid cavitation, consumes noticeable amount of power, and requires maintenance expenses to handle reliability issues.

The power consumed by the pump is a deductible from the power obtained in the expansion machine, and reduces the overall refrigerant system thermodynamic efficiency.

Usually the required refrigerant subcooling is provided by elevating the condenser pressure, which also reduces the amount of generated power and thermodynamic efficiency of the refrigerant system. In addition, having subcooled refrigerant at the outlet of the condenser unit is associated with additional refrigerant charge and increased size of the condenser unit. In other words, all these factors increase the system cost.

Pumping capacity adjustments are typically provided by variable speed drives or any other known capacity reduction methods, such as throttling or pulse width modulation controlled valves. This also adds cost, reduces thermodynamic efficiency and may introduce reliability issues. Similarly, if bypass technologies are applied to reduce pumping capacity, the system efficiency and operating cost will suffer.

International application number PCT/US97/20229 published under the Patent Cooperation Treaty (International Publication Number WO99/24766) discloses a solar powered heating and cooling system containing a high temperature heat source with an arrangement to allow for a low-pressure liquid to flow from a condenser to a vaporizer by way of gravity. However, although the concepts related to refrigerant flows driven by gravity are known in the art, this application doesn't disclose or suggest any particular component design, system configuration, valve arrangement or any other means of how this can be accomplished.

SUMMARY OF THE INVENTION

Briefly, in accordance with one aspect of the invention, a Rankine system comprises a closed-loop refrigerant cycle with an expansion machine, a condenser unit, a gravity-driven pumping unit, and a boiler unit. The gravity driven pumping unit has an inlet valve, an outlet valve, and a staging zone therebetween. The inlet valve is connected to the condenser unit and the outlet valve is connected to the boiler unit. The condenser unit is located above the boiler unit, with respect to gravity direction. The inlet valve, the outlet valve, the liquid line and entire path established between the condenser and boiler units are oriented progressively downwards, with respect to gravity direction, and are sized and shaped to allow for vapor refrigerant to freely move upward from the boiler unit to the condenser unit, and to allow for liquid refrigerant to freely drain downwards from the condenser unit to the boiler unit by way of gravity. The control system facilitates operation of the gravity-driven pumping unit by opening and closing of the inlet and outlet valves in a sequence, which enables gravity-driven movement of liquid refrigerant from the condenser unit to the staging zone and then from the staging zone to the boiler unit, against a positive pressure differential between the boiler and condenser units.

The gravity-driven pumping unit does not require substantial subcooling at the pump inlet, and this is another aspect of the invention that overcomes design and operation difficulties associated with the prior art.

In yet another aspect of the invention, the control system operates with a timer to sequentially fill the staging zone with the refrigerant during one time interval and to subsequently discharge the refrigerant from the staging zone during another time interval. Further, there could be a time delay prior to opening the outlet valve and a time delay prior to opening the inlet valve incorporated into the control logic. The control system assigns normal values to the time intervals to provide maximal pumping capacity, and changes time intervals to decrease pumping capacity. A plurality of gravity driven pumping units may be used in combination with each other.

In yet another aspect of the invention, a Rankine system with a gravity-driven pump has a number of boiling pressure levels. The expansion machine has a single inlet associated with the highest boiling pressure level, and a number of other intermediate inlets introducing refrigerant streams into the expansion processes that are associated with other intermediate boiling pressure levels.

In yet another aspect of the invention, a Rankine system with a gravity-driven pump has a condenser unit with a number of condenser sections connected in sequence. Each condenser section is feeding one gravity-driven pumping unit with the refrigerant liquid and feeding a next downstream condenser (if any) with the refrigerant vapor.

In the drawings as hereinafter described, preferred and alternate embodiments are depicted; however various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a Rankine system with a gravity-driven pump, in accordance with the present invention;

FIG. 2 is a graphic illustration of the timing sequence in the operation thereof;

FIGS. 3A and 3B are schematic illustrations of a coaxial valve in closed and in opened positions, respectively;

FIG. 4 is a staging zone with an in-line pressure relief device, in accordance with the present invention;

FIG. 5A-5D are gravity-driven pumps with valve assemblies made of two adjacent solenoid valves with different normal flow directions, in accordance with the present invention;

FIG. 6 is a schematic illustration of a Rankine system with a plurality of gravity-driven pumps, in accordance with the present invention;

FIG. 7 is a graphical illustration of the timing sequence of the control logic for the operation of a gravity-driven pumping unit with a plurality of pumps;

FIG. 8 is a schematic illustration of a Rankine system with two boiling pressure levels, in accordance with the present invention;

FIGS. 9A and 9B are schematic illustrations of a two-stage expansion machine with two turbines connected in sequence and two turbines connected in parallel, respectively;

FIG. 10 is a schematic illustration of a Rankine system with a gravity-driven pump as providing cogeneration of thermal and mechanical energy, in accordance with the present invention;

FIG. 11 is a schematic illustration of a Rankine system with a gravity-driven pump and staged condensation, in accordance with the present invention;

FIG. 12 is a schematic illustration of a Rankine system with a gravity-driven pump, two boiling pressure levels, and staged condensation, in accordance with the present invention;

FIG. 13 is a schematic illustration of a two-stage condensation coil with one pass in each condensation stage, in accordance with the present invention;

FIG. 14 is a schematic illustration of a two-stage condensation coil with two passes in the first condensation stage and one pass in the second condensation stage;

FIG. 15 is a schematic illustration of a two-stage condensation coil with two passes in the first condensation stage and three passes in the second condensation stage;

FIG. 16 is a schematic illustration of a two-stage condensation coil with five passes in the first condensation stage and four passes in the second condensation stage;

FIG. 17 is a schematic illustration of a three-stage condensation shell-and-tube heat exchanger with vertical baffles;

FIG. 18 is a schematic illustration of a three-stage condensation shell-and-tube heat exchanger with horizontal baffles;

FIG. 19 is a schematic illustration of a combined vapor compression and Rankine cycle with a gravity-driven pump.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, a Rankine system with a gravity-driven pump includes a condenser unit 1, a gravity-driven pumping unit 2, a boiler unit 3, and an expansion machine 4.

The condenser unit 1 provides thermal contact and heat transfer interaction between a fluid to be heated (e.g. air, water, or brine) and refrigerant vapor to be condensed. The condenser unit 1 delivers a subcooled liquid at the condensing pressure P₁ at the condenser outlet.

The gravity-driven pumping unit 2 is installed on a liquid line 5, which connects the condenser unit 1 and the boiler unit 3 through the pumping unit 2.

The boiler unit 3, which provides thermal contact and heat transfer interaction between a fluid carrying enthalpy of available thermal energy and refrigerant vapor to be boiled, delivers superheated vapor at the boiling pressure P₂, which is higher than the condensing pressure P₁.

The expansion machine 4, for instance of a turbine, scroll, screw, reciprocating, rotary or any other type, expands refrigerant vapor and produces useful mechanical work. A high-pressure vapor line 6 connects an outlet from the boiler unit 3 and an inlet to the expansion machine 4. A low-pressure vapor line 7 connects an outlet from the expansion machine 4 and an inlet to the condenser unit 1.

The gravity-driven pumping unit 2 has an inlet valve 8, a staging zone 9, and an outlet valve 10. The inlet valve 8 is connected to a source of liquid refrigerant, which is, in this case, the condenser unit 1. The outlet valve 10 is connected to the boiler unit 3. The condenser unit 1 is located above, with respect to gravity (i.e. at a higher elevation), the boiler unit 3. The liquid line 5 and the gravity-driven pumping unit 2 are oriented downwardly (vertically or inclined) to enable operation of the gravity-driven pumping unit 2.

The gravity driven pump may have a receiver 55 located upstream of the inlet valve 8. Also, the receiver may be a part of the condenser unit 1.

Utilizing the Archimedes and gravity forces as described below, the gravity-driven pumping unit 2 receives liquid refrigerant at the condensing pressure P₁ from the condenser unit 1 and pumps it into the boiler unit 3, where the boiling pressure P₂>P₁ is maintained. In the boiler unit 3, liquid refrigerant is boiled due to heat transfer interaction with the fluid carrying enthalpy of available thermal energy. From the boiling unit 3, superheated vapor enters the expansion machine 4 through the high pressure vapor line 6, is expanded there from the boiling pressure P₂ to the condensing pressure P₁, producing useful mechanical energy that can be obtained from the shaft of the expansion machine 4. The generated mechanical work may be transferred into electrical power or may be directly applied to other mechanically driven devices. The expanded refrigerant vapor, arriving in the condenser unit 1 through the low-pressure vapor line 7, is desuperheated, condensed, and subcooled at the condensing pressure P₁, due to heat transfer interaction with the fluid to be heated. Liquid from the condenser unit 1 is pumped through the liquid line 5 by the gravity-driven pumping unit 2, and the sequence of the thermal processes of the Rankine cycle is repeated.

Operational principles of the gravity-driven pumping unit 2 shown in FIG. 1 are illustrated in FIG. 2, which consists of graphs representing an idealized pressure diagram, position diagram for the inlet valve 8, and a position diagram for the outlet valve 10, with respect to time. The pressure diagram indicates changes in pressure in the staging zone 9, with respect to pressure P₁ in the condenser unit 1 and pressure P₂ in the boiler unit 3. The valve position diagrams indicate opened and closed positions of the inlet and outlet valves 8 and 9.

Initially, the inlet valve 8 is opened and the outlet valve 10 is closed. This facilitates the filling process. A portion of the vapor refrigerant from the staging zone 9 moves upward to the condenser unit 1, due to the Archimedes force, while the liquid refrigerant from the condenser unit 1 drains downwards to the staging zone 9, due to the gravity effect, at a relatively slow drainage rate. Thus, the drained portion of liquid refrigerant replaces the vapor refrigerant in the staging zone 9. During the filling process, pressure in the staging zone 9 becomes equal to the pressure P₁ of the condenser unit 1 as shown on FIG. 2.

Next, the inlet valve 8 is closed, and there is no vapor or liquid flow associated with the staging zone 9, since the inlet valve 8 and the outlet valves 10 are closed.

When the outlet valve 10 is opened, the staging zone 9 and the boiler unit 3 are brought into communication. Liquid refrigerant in the staging zone 9 is pressurized by the vapor in the boiler unit 3, so that pressure in the staging zone 9 becomes equal to the pressure P₂ in the boiler unit 3, and the discharge process is initiated. After the pressure equalization, a portion of the vapor refrigerant moves upward from the boiler unit 3 to the staging zone 9, due to the Archimedes force, and the liquid refrigerant from the staging zone 9 drains downwards to the boiler unit 3, due to the gravity effect. The drained liquid refrigerant is boiled in the boiler unit 3. The pressure diagram on FIG. 2 demonstrates the pressure elevation to the value P₂ in the staging zone 9.

Next, the outlet valve 10 is closed and there is no vapor or liquid flow associated with the staging zone 9. Opening of the inlet valve 8 initiates the filling process again, and the above-described gravity-driven pumping cycle is repeated.

A design challenge of the gravity-driven pumps is a consideration of the impact of a wall temperature of the staging zone 9. The wall temperature is established as a result of thermal interaction with the ambient environment, liquid refrigerant received from the condenser unit 1, vapor refrigerant received from the boiler unit 3, and the result of thermal bridges between the condenser unit and the staging zone 9, and between the boiler unit 3 and the staging zone 9. If pressure in the staging zone 9 appears to correspond to the saturation conditions at the wall temperature, the condensation of vapor inside the staging zone during the discharge process takes place. A portion of the vapor refrigerant from the boiler unit 3 moves upwardly, replaces the drained liquid in the staging zone 9, and is condensed there at a certain condensation rate when it contacts the staging zone wall. Liquid refrigerant drains downwardly to the boiler unit 3 at a certain, relatively low drainage rate. The condensation rate reduces the amount of refrigerant delivered from the boiling unit 3 and ultimately may be equal to the liquid drainage rate. The condensation process is terminated when the liquid refrigerant is sufficiently heated up by the refrigerant vapor that is moved up by the Archimedes force. Insulating of the gravity-driven pump may reduce the rate of the condensation process and improve the pump efficiency.

It is appropriate to introduce volumetric efficiency of the staging zone 9 as a ratio

$\begin{matrix} {{\eta_{v} = {\frac{m_{a} - m_{0}}{m_{\max}} = \frac{{\rho^{\prime}\left( t_{a} \right)} - {\rho \left( {p_{2};t_{2}} \right)}}{\rho^{\prime}\left( t_{amb} \right)}}},} & (1) \end{matrix}$

where η_(v)—volumetric efficiency of the staging zone; m_(a)—actual mass of liquid refrigerant filled the staging zone 9 at actual refrigerant temperature t_(a); m₀—mass of refrigerant vapor remained in the staging zone 9, defined at the boiler pressure p₂ and temperature t₂, prior to closing of the outlet valve 10; m_(a)-m₀—mass of refrigerant pumped to the boiler unit 3 during one pumping cycle; m_(max)—is maximal mass of liquid refrigerant filling the staging zone when temperature of refrigerant inside the staging zone is equal to ambient temperature t_(aamb); ρ′(t_(a))—density of saturated liquid refrigerant filled the staging zone at actual refrigerant temperature t_(a); ρ(p₂;t₂)—density of refrigerant vapor at temperature t₂ and pressure p₂; ρ′(t_(amb))—density of saturated liquid refrigerant at temperature t_(amb) of ambient environment.

Volumetric efficiency of the staging zone 9 is reduced if a portion of liquid refrigerant remains in the staging zone 9. Longer opening of the outlet valve 10 reduces the amount of liquid refrigerant remaining in the staging zone, but extends the discharge time and reduces the pumping capacity.

The lower the wall temperature of the staging zone 9 is, the lower the temperature of liquid refrigerant filling the staging zone 9 is, the higher the liquid refrigerant density is, and the greater the liquid refrigerant mass m_(a) filling the staging zone 9 is. This causes volumetric efficiency to increase. On the other hand, the lower the wall temperature of the staging zone 9 is, the higher the condensation rate is, and the larger the mass m₀ is. This causes volumetric efficiency to decrease.

Oppositely, the higher the wall temperature of the staging zone 9 is, the higher the temperature of liquid refrigerant filling the staging zone 9 is, the lower the liquid refrigerant density is, the lower the liquid refrigerant mass m_(a) filling the staging zone 9 is. This causes volumetric efficiency to decrease. On the other hand, the higher the wall temperature of the staging zone 9 is, the lower the condensation rate is, and the smaller the mass m₀ is. This causes a volumetric efficiency to increase.

Maximal volumetric efficiency is achieved when wall temperature of the staging zone 9, during the filling process, is equal to the ambient temperature and the wall temperature of the staging zone 9, during the discharge process, is equal to the boiler temperature. However, taking into account that ambient temperature is close to the condenser temperature, the best practical compromise is obtained when the staging zone 9 is placed as close to the boiler unit 3 as possible. In this case, during the discharge process, the wall temperature of the staging zone 9 is established as close to the boiler temperature as possible, due to the thermal conductivity of the wall material. During the filling process, the wall temperature of the staging zone 9 is established as close to the ambient temperature as possible, due to high specific capacity of liquid refrigerant filling the staging zone 9.

If ambient temperature is close to boiling temperature, the best practical compromise is obtained when the staging zone 9 is placed as close to the condenser unit 3 as possible.

Vertical orientation reduces the wall temperature effect, in comparison with inclined orientations.

Operation of the gravity-driven device in the above embodiments implies the use of two-way solenoid valves. Conventional solenoid valves are the devices that stop fluid flow against a rated pressure differential in one direction, which is a normal flow direction. Usually, they do not stop flow in the opposite direction. Solenoid two-way valves that stop fluid flow in both directions are called bi-directional valves. If rated pressure differentials are different for each direction, the direction, which is rated for a higher pressure differential is called a normal flow direction. Otherwise, a normal flow direction does not exist.

In order to efficiently provide the pumping duty, the gravity-driven pump should meet the following requirements: 1) the inlet valve 8 and the outlet valve 10 should have the ability to stop refrigerant flow in the direction from the boiler unit 3 to the condenser unit 1; 2) at least one valve should have the ability to stop refrigerant flow in both directions (that is, at least one valve should be a bi-directional flow control device); 3) internal ports of the inlet valve 8 and outlet valve 10, and internal dimensions of the liquid line 2 should be sized and shaped to allow for refrigerant vapor to flow upwardly, due to the Archimedes force, and for liquid refrigerant to flow downwardly, due to the gravity force; and 4) the orientation of the path inside the inlet valve 8, the outlet valve 10, the liquid line 2, and a line connecting the outlet valve 10 and the boiler unit 13 should allow refrigerant vapor to flow upwardly, due to the Archimedes force and liquid refrigerant to flow downwardly, due to the gravity force.

Gravity-driven pumps are operational when the inlet valve 8 is a conventional normally open solenoid valve and the outlet valve 10 is a normally closed bi-directional solenoid valve. Alternatively, gravity-driven pumps may operate when the outlet valve 10 is a conventional normally open solenoid valve and the inlet valve 8 is a normally closed bi-directional solenoid valve. If no valve is normally open, the trapped liquid may boil out during off-cycle and destroy the gravity-driven pump.

Conventional solenoid valves are usually either direct-acting or pilot-operated devices. The direct-acting solenbid valves have the ports that are too small to be applicable here. An increase of the port size is associated with an increase of force keeping the valve seat in an appropriate position, since the force is proportional to the valve port area. The coil actuating the valve limits the force. The pilot-operated valve uses available pressure to keep the valve seat in an appropriate position. Although this operational principle significantly reduces the force, the pilot-operated valves are one-directional devices only.

An example of the bi-directional valve is a coaxial valve, as is shown in FIGS. 3A and 3B. As shown in FIG. 3A the coaxial valve consists of a casing 11, a seat 12 (which is not a moving part), and a hollow tube 13 (which is a moving part). The area between the casing 11 and the seat 12 is a cross-section of an inlet port 14. An outlet port 15 is located at the opposite end. The hollow tube 13 has sealing rings 16 between the hollow tube 13 and the casing 11.

When the hollow tube 13 is positioned against the seat 12, creating a seal, the co-axial valve is in the closed position, as shown in FIG. 3A. In the closed position, the valve stops the flow from the inlet port 14 to the outlet port 15, and from the outlet port 15 to the inlet port 14. When the hollow tube 13 is moved to other end, as shown in FIG. 3B, the co-axial valve is in the open position. In the open position, the valve allows refrigerant streams to flow from the inlet port 14 to the outlet port 15, and from the outlet port 15 to the inlet port 14, as indicated by the arrows in FIG. 3B. The force moving the hollow tube 13 to or from the seat 12, or keeping the hollow tube 13 against the seat 12, is not proportional to the port size, therefore the port size may be as large as needed.

Hollow tubes of co-axial valves have short strokes between the open and closed positions. Therefore, sizing of the co-axial valves for the gravity-driven pump should be based on either the cross-sectional area around the seat 12 or the cross-sectional area between the seat 12 and the hollow tube 13 in the open position, whichever is smaller. The internal diameter of the hollow tube 13 is usually larger than those cross-sectional areas.

Motorized ball valves and modulation valves actuated by stepper motors may perfectly meet all four requirements stated above. However, since the position of these valves cannot be controlled when power is off, liquid may be trapped between the inlet valve 8 and the outlet valve 10. The trapped liquid may cause dangerous pressure elevation in the staging zone 9 when the temperature around the zone and inside the zone is raised during the off-cycle. In this case, an in-line pressure relief device 9 a connecting the staging zone 9 with any point of the Rankine system, outside the staging zone 9, and preferably to a point on the low pressure side, shall be provided, as shown on FIG. 4.

FIGS. 5A-5D show options to use conventional solenoid valves in gravity-driven pumps. It is assumed that the third and the fourth requirements mentioned above are provided.

In FIG. 5A, the inlet valve 8 is a conventional solenoid valve installed to provide normal flow direction from the boiler unit 3 to the condenser unit 1. The outlet valve arrangement provides bi-directional operation and is configured of two adjacent conventional valves, which are a first valve 10 a and a second valve 10 b. The first valve 10 a is a conventional solenoid valve installed to provide normal flow direction from the condenser unit 1 to the boiler unit 3. The second valve 10 b is a normally closed conventional solenoid valve installed to provide normal flow direction from the boiler unit 3 to the condenser unit 1. If the inlet valve 8 in FIG. 5A is a normally open solenoid valve, the first valve 10 a may be either normally open or normally closed solenoid valve; the second valve 10 b should be a normally closed solenoid valve. If the inlet valve 8 in FIG. 5A is a normally closed solenoid valve, the first valve 10 a should be a normally open solenoid valve; the second valve 10 b may be either a normally open or normally closed solenoid valve.

The first valve 10 a in FIG. 5B is a conventional solenoid valve installed to provide normal flow direction from the boiler unit 3 to the condenser unit 1. The second valve 10 b is a conventional solenoid valve installed to provide normal flow direction from the condenser unit 1 to the boiler unit 3. If the inlet valve 8 in FIG. 5B is a normally open solenoid valve, the first valve 10 a should be a normally closed solenoid valve; the second valve 10 b may be either a normally open or normally closed solenoid valve. If in FIG. 5B the inlet valve 8 is a normally closed solenoid valve, the first valve 10 a should be a normally opened solenoid valve; the second valve 10 b may be either a normally open or normally closed solenoid valve.

Operational principles of gravity-driven pumps with adjacent conventional solenoid valves are the same as the operational principles of the gravity-driven pump as shown in FIG. 1, except that the opening and closing of adjacent valves happens simultaneously.

FIG. 5C has the inlet valve arrangement providing bi-directional operation and is made of two adjacent conventional valves, which are a first valve 8 a and a second valve 8 b. The first valve 8 a is a conventional solenoid valve installed to provide normal flow direction from the condenser unit 1 to the boiler unit 3. The second valve 8 b is a conventional solenoid valve installed to provide normal flow direction from the boiler unit 3 to the condenser unit 1. If the outlet valve 10 is a normally closed solenoid valve, the first valve 8 a and the second valve 8 b may be either normally closed or normally open solenoid valves. If the outlet valve 10 as a normally open solenoid valve, the first valve 8 a may be either a normally closed or normally open, but the second valve 8 b should be a normally closed solenoid valve.

In FIG. 5D, the first valve 8 a is a conventional solenoid valve installed to provide normal flow direction from the boiler unit 3 to the condenser unit 1. The second valve 8 b is a conventional solenoid valve installed to provide normal flow direction from the condenser unit 1 to the boiler unit 3. If the outlet valve 10 is a normally closed solenoid valve, the first valve 8 a and the second valve 8 b may be either normally closed or normally open. If the outlet valve 10 is a normally open solenoid valve, the second valve 8 b may be either a normally closed or normally open valve, but the first valve 8 a should be a normally closed solenoid valve.

In accordance with FIG. 1, the gravity-driven pumping unit 2 of the Rankine system has one gravity-driven pump. When the pump discharges a portion of liquid refrigerant into the boiler unit 3, the boiling process begins and a certain boiling pressure is established. During the boiling process, the amount of liquid in the boiling unit 3 is reduced. The reduced amount of boiling liquid refrigerant causes a reduced amount of generated refrigerant vapor. As a result, the boiling pressure is reduced. This causes a reduction of condensing pressure and rotating speed of the expansion machine 4. When a new portion of liquid refrigerant arrives, the boiling and condensing pressures and rotating speed are recovered and the pumping cycle is repeated. Thus, fluctuations of the boiling and condensing pressures as well as in a rotating speed of the expansion machine 4 between pumping cycles are taking place.

In order to reduce fluctuations of pressures and in a rotating speed and provide continuous pumping operation, a plurality of gravity-driven pumps is used. Acceptable levels of pressures and rotating speed fluctuations dictate a number of gravity-driven pumps.

The Rankine system having a plurality of gravity driven devices is shown in FIG. 6. A gravity driven pumping unit 2 consists of a first gravity driven pump 2 a, a second gravity driven pump 2 b, and a third gravity driven pump 2 c operating in parallel. A liquid refrigerant receiver 55 is installed at the inlet to the gravity driven pumping unit 2 to ensure availability of liquid at the inlet to the pumping unit. A control system 112 regulates operation of the gravity-driven pumps 2 a, 2 b, and 2 c.

Control system 112 regulates the following sequence of operation for each gravity-driven pump: opening the inlet valve 8, allowing a sufficient time interval to fill the staging zone 9 with liquid refrigerant, closing the inlet valve 8, allowing a sufficient time delay prior to opening of the outlet valve 10, opening the outlet valve 10, allowing a sufficient time interval to discharge refrigerant from the staging zone 9, closing the outlet valve 10, allowing a sufficient time delay prior to opening of the inlet valve 8, and continuously repeating this sequence, which is illustrated by FIG. 2.

In accordance with the sequence described above, the pumping capacity of each gravity-driven pump depends on a filling time interval τ_(f), which is the time of filling the staging zone 9 with liquid refrigerant; a discharging time interval τ_(d), which is the time of discharging refrigerant from the staging zone 9 to the boiler unit 3; the time delay τ₁ prior to opening the inlet valve 8 (including time of the opening); and the time delay τ₂ prior to opening the outlet valve 10 (including time of the opening).

The control logic of the control system 112 is shown in FIG. 7, which illustrates the sequence of opening and closing the inlet and outlet valves and the time intervals τ_(f), τ_(d), τ₁, and τ₂ mentioned above.

Let us define the pumping cycle as a process utilizing one discharging action. In accordance with FIG. 7, the duration of the pumping cycle of one gravity-driven pumping device is equal to

τ₀=τ_(f)−τ_(d)−τ₁+τ₂  (3)

When a number of gravity-driven pumps operate in sequence, one discharge operation happens during time calculated as

τ₀=τ_(d)+τ₃,  (4)

where τ₃—is the time interval between the closing of an outlet valve of one gravity-driven pump and the opening of an outlet valve of another gravity-driven pump (including the closing and opening times), which operates in a sequential order, with respect to the first pump. In FIG. 7, it is shown that τ₃=τ₂; however, this particular relationship is not required.

The time calculated per formula (4) implies that there are to be a number of pumps operating in sequential order

$\begin{matrix} {{n = {r \cdot \frac{\tau_{f} + \tau_{d} + \tau_{1} + \tau_{2}}{\tau_{d} + \tau_{3}}}},} & (5) \end{matrix}$

where r—is a correction factor adjusting n to an integer value; for instance, it may happen that r=1.

Each sequential step may include a number of pumps operating in parallel, with either simultaneous or overlapping cycles. In general, the mass flow rate provided by the gravity-driven pumping unit is

$\begin{matrix} {{G = {\frac{\left( {m_{a} - m_{0}} \right) \cdot k \cdot n}{\tau_{0}} = \frac{\eta_{v} \cdot m_{\max} \cdot k \cdot n}{\tau_{0}}}},} & (6) \end{matrix}$

where k—is a number of gravity-driven pumps operating in parallel at the moment.

At certain filling and discharge times (τ_(f)=τ_(f0) and τ_(d)=τ_(d0)), and when time delays prior to opening inlet and outlet valves 8 and 10 are minimal (τ₁→0 and τ₂→0), the mass flow rate provided by one gravity-driven device is at its maximum. These times are called nominal times. The mass flow rate is reduced when τ_(f)≠τ_(f0), τ_(d)≠τ_(d0), τ₁>0 and τ₂>0. If τ_(f)<τ_(f0) or τ_(d)<τ_(d0), the flow capacity is reduced, because the filling process or the discharge process is incomplete. If τ_(f)>τ_(f0) or τ_(d)>τ_(d0), the mass flow rate is reduced, because the pumping cycle duration is increased. For the same reason, the mass flow rate is reduced when time delays τ₁ and τ₂ are increased.

The same conclusions are applicable for a plurality of gravity driven pumps, even though formula (4) includes τ_(d) and τ₃ only. This is because time τ₃ depends on time τ_(f), τ_(d), τ₁, and τ₂, in accordance with formula (5). Also, time τ₃ depends on the number of gravity-driven pumps n operating in a sequential order. Thus, having a plurality of gravity-driven pumps, allows an additional option to engage a different number of pumps, in order to change the pumping capacity. The changed number of pumps may need changing time τ_(f), τ_(d), τ₁, or τ₂.

A control system 112 makes adjustments of the mass flow rate with time τ_(f), τ_(d), τ₁, and τ₂ based on readings from a temperature sensor 113 and a pressure sensor 114. The boiler unit 3 is sized to maintain a certain nominal superheat at maximal flow capacity. If the refrigerant superheat, as monitored by the temperature sensor 113 and the pressure sensor 114 is decreased, the control system 112 decreases the refrigerant mass flow rate. If the superheat is increased, the control system 112 increases the refrigerant mass flow rate.

The gravity-driven pumping unit may be operated in a pressure relief mode. If pressure on the high-pressure side of the Rankine system is undesirably increased, based on readings as provided by the pressure sensor 114, the control system 112 opens the inlet valve 8 and the outlet valve 10 and releases the undesirably increased refrigerant pressure into the condenser unit 1.

The higher the pressure at the inlet to the expansion machine 4 is, the higher the potential efficiency of the Rankine cycle is. On other hand, the higher the boiling pressure is, the higher the temperature of the fluid at the outlet from the boiler unit 3 is, and the lower the extent of utilization of the thermal energy is.

In FIG. 8, a boiler unit 3 is comprised of a first boiler 3 a operating at a high boiling temperature and a second boiler 3 b operating at a low boiling temperature. A gravity-driven pump 2 a feeds the first boiler 3 a and a gravity-driven pump 2 b feeds the second boiler 3 b. A fluid 115 carrying thermal energy is cooled in the first boiler 3 a to an intermediate temperature and is further cooled in the second boiler 3 b to a temperature approaching the low boiling temperature. Refrigerant exiting the first boiler 3 a feeds a main inlet 116 of the expansion machine 4. Refrigerant from the second boiler 3 b feeds an intermediate inlet 117 to the expansion machine 4, introducing a portion of refrigerant at a low boiling pressure into the expansion process. Thus, the expansion machine 4 is fed through the main inlet with the same amount of vapor refrigerant at a high boiling pressure as a boiler operating in the Rankine cycle with one boiling pressure level, and, at the same time, the low boiling temperature allows an extraction of power from the thermal energy source to a greater extent. As a result, efficiency of the Rankine system is significantly improved.

Ultimately, the Rankine system may have a number of boiling pressure levels, and the same number of boilers, inlets to the expansion machine 4, and gravity-driven pumping units.

Alternatively to the expansion machine 4 with the main inlet 116 and the intermediate inlet 117, a two-stage (or multistage) expansion machine 4 having two turbines or two expanders 4 a and 4 b may be used as shown in FIGS. 9A and 9B.

FIG. 9A relates a two-stage expansion machine 4 with a first stage 4 a and a second stage 4 b connected in sequence. Each stage 4 a and 4 b may have a plurality of turbines or other expansion devices. Refrigerant vapor at a high boiling pressure enters the first stage 4 a through an inlet 116. The first stage 4 a expands the entered portion of refrigerant to an intermediate pressure equal to a low boiling pressure. The expanded portion of refrigerant at the low boiling pressure is mixed with portion of refrigerant incoming through the intermediate inlet 117 from the second boiler, similarly to the Rankine system in FIG. 7. Further expansion to a condensing pressure is executed in the second stage 4 b. Ultimately, the expansion machine 4 may have a number of turbines or other expansion devices, such as scrolls, screws or reciprocating pistons, connected in sequence, and the same number of gravity-driven pumping units, boiling pressure levels and boilers.

In FIG. 9B, a first stage 4 a and a second stage 4 b operate in parallel. Each stage 4 a and 4 b may have a plurality of turbines or other expansion devices. Refrigerant vapor at a high boiling pressure enters the first stage 4 a through an inlet 116 and is expanded to a condensing pressure. Refrigerant vapor at a low boiling pressure enters the second stage 4 b through an intermediate inlet 117 from the second boiler, similarly to the Rankine system in FIG. 7, and is expanded to a condensing pressure as well. Ultimately, the expansion machine 4 may have a number of turbines or other expansion devices, such as scrolls, screws or reciprocating pistons, connected in parallel, and the same number of gravity-driven pumping units, boiling pressure levels and boilers.

In both cases shown on FIG. 9A and FIG. 9B, the two expansion stages may be attached to one shaft, or they may have independent shafts with independent distribution of the recovered mechanical energy.

Two levels of boiling pressure may be utilized in the Rankine system providing co-generation of thermal and mechanical energy, as shown in FIG. 10. The Rankine system recovers energy in the expansion machine 4 and, at the same time, it executes heating duties in a condenser unit 1 a and condenser unit 1 b through the thermal and heat transfer interaction with the fluid 115. The condenser unit 1 a heats a fluid 118 a providing high quality thermal output; the condenser unit 1 b heats a fluid 118 b providing low quality thermal output. Optionally, the condenser units 1 a and 1 b may heat a single fluid with two steps of heating 118 a and 118 b.

Thermal energy is absorbed in a boiler unit 3 at two boiling pressure levels. A high boiling pressure is maintained in a boiler 3 b, which feeds the expansion machine 4 through an inlet 116. A low boiling pressure is maintained in a boiler 3 a, which feeds the expansion machine 4 through an intermediate inlet 117. The boiler 3 a is fed by a gravity-driven pumping units 2 b, and the boiler 3 b is fed by gravity-driven pumping units 2 a and 2 c.

Refrigerant condensation occurs at two pressure levels as well. The condenser unit 1 a operates at a condensing pressure which corresponds to a pressure at the outlet of the expansion machine 4. The condenser unit 2 a operates at a pressure which is equal to the low boiling pressure.

The condenser unit 1 a feeds the gravity-driven pumping units 2 a and 2 b, or alternatively may feed the gravity-driven pumping unit 2 b only. The condenser unit 2 a feeds the gravity-driven pumping units 2 c.

Ultimately, the Rankine system providing co-generation of thermal and mechanical energy has a number of condensers and condensing pressure levels and the same number of boilers, boiling pressure levels, and inlets to the expansion machine 4 (or expansion machines as shown in FIGS. 9A and 9B). The number of gravity-driven pumping units should be at least the same; however, it may vary. For example, if there are n pressure levels: P₁, P₂, P₃, P_(N-1), and P_(N), where P₁ is the lowest pressure level and is a condensing pressure only; P_(N) is the highest pressure level and is the boiling pressure only. The number of boiling pressures is n−1: P₂, P₃, . . . , P_(N-1) and P_(N). The number of condensing pressures is the same and they are: P₁, P₂, P₃, . . . , and P_(N-1). Thus, pressures P₂, P₃, . . . , and P_(N-1) are boiling pressures and, at the same time, they are condensing pressures. A condenser operating at the pressure level P₁ may have n−1 pumps, a condenser operating at the pressure level P₂ may have n−2 pumps, a condenser operating at the pressure level P₃ may have n−3 pumps, and so on. Thus, n pressure levels ultimately allow for

$\sum\limits_{i = 1}^{n - 1}i$

pumps.

It is known that liquid refrigerant condensed inside refrigerant channels occupies an insignificant portion of the entire internal condenser unit volume, but it is primarily positioned at the condenser unit walls and covers up a significant portion of internal heat transfer area. As a result, vapor refrigerant, which occupies a significant part of the entire internal volume, does not contact the condenser unit walls, and overall heat transfer coefficient is substantially reduced. Removal of the condensed refrigerant from the condenser unit may significantly improve performance characteristics of the entire system.

FIG. 11 relates to the Rankine system with staged condensation. The condenser unit 1 includes a first condenser 1 a, a second condenser 1 b, and a third condenser 1 c cooled by a fluid 118, which could be, for instance, water, air, or brine. Each condenser feeds its own receiver and gravity-driven pump. A first receiver 55 a and a first gravity-driven pump 2 a are associated with the first condenser 1 a, a second receiver 55 b and a second gravity-driven pump 2 b are associated with the second condenser 1 b, and a third receiver 55 c and a third gravity-driven pump 2 c are associated with the third condenser 1 c. The first, second, and third pumps 2 a, 2 b, and 2 c may include pluralities of pumps.

Refrigerant vapor exiting the expansion machine 4 is partially condensed in the third condenser 1 c. The condensed liquid portion is directed into the third receiver 55 c and the remaining portion of the refrigerant vapor enters the second condenser 1 b, where it is partially condensed. Subsequently, the condensed liquid portion is routed into the second receiver 55 b and the remaining portion of the refrigerant vapor enters the first condenser unit 1 c. In the first condenser 1 c, the refrigerant is completely condensed and then fills the first receiver 55 a.

FIG. 12 is another illustration of utilization of the staged condensation, although combined with two boiling pressure levels. The condenser unit 1 comprises a first condenser 1 a and a second condenser 1 b and is cooled by a fluid 118, which could be, for instance, water, air, or brine. The boiler unit 3 consists of a first boiler 3 a and a second boiler 3 b, heated by a fluid 115 carrying enthalpy of available thermal energy. Each condenser feeds its own receiver, pump, and boiler. A first receiver 55 a, a first gravity-driven pump 2 a, and a first boiler 3 a are associated with the first condenser 1 a. A second receiver 55 b, a second gravity-driven pump 2 b, and a second boiler 3 b are associated with the second condenser 1 b. This system requires an expansion machine 4 with two inlets, or an expansion machine 4 as shown in FIGS. 9A and 9B.

This system combines the advantages of two levels of boiling pressure, which improve efficiency of the Rankine system, and removal of liquid from the condensation process, which improves performance of the condensers and, ultimately, efficiency of the entire Rankine system.

There are different opportunities in providing staged condensation in the condenser units.

FIGS. 13-16 relate to air-cooled condenser units. Each condensation stage may be circuited to have a number of passes.

FIG. 13 shows a two-stage condenser unit with one pass in each stage. The condenser unit has an inlet header 24, an outlet header 25, and plurality of refrigerant channels 26 extending between the inlet and outlet headers 24 and 25. The refrigerant channels 26 are sealed within the inlet and outlet headers 24 and 25. The external surface of the channels is thermally exposed to a cooling fluid. The inlet header 24 has a vapor inlet 27 and a liquid outlet 29. The outlet header 25 has an intermediate liquid outlet 28. The inlet header 24 contains a baffle 30 splitting it into two portions 31 and 32 and routing the condensing refrigerant stream into two passes 33 and 34. One portion is associated with the pass 33 and the vapor inlet 27; another portion is associated with the pass 34 and the liquid outlet 29.

While the condenser unit in FIG. 13 has only one pass in each condensation stage, FIG. 14 presents a condenser unit having two passes 33 a and 33 b in a first condensation stage 33 and one pass in a second condensation stage 34. An inlet header 24 has a phase separator 30. The phase separator 30 splits the inlet header 24 into an upper chamber 31 associated with the vapor inlet 27 and a lower chamber 32 associated with an intermediate outlet 28. An outlet header 25 has a phase separator 35, which splits the outlet header into an upper chamber 36 and a lower chamber 37. The upper chamber 36 is associated with a first condensation stage 33. The lower chamber 37 is associated with a second condensation stage 34 and a liquid outlet 29.

It is possible to have a condenser unit with multiple passes in each condensation stage. For example, FIG. 15 shows two passes 33 a and 33 b in a first condensation stage 33 and three passes 34 a, 34 b, and 34 c in a second condensation stage 34. Phase separators 30 and 36 in an inlet header 24, and phase separators 35 and 37 in an outlet header 25, are employed. Also, a collector 29 a is employed near a liquid outlet 29.

FIG. 16 shows five passes 33 a, 33 b, 33 c, 33 d, and 33 e in a first condensation stage 33 and three passes 34 a, 34 b, and 34 c in a second condensation stage 34. Phase separators 30, 36, 38, and 40 in an inlet header 24 and phase separators 35, 37, 39, and 41 in an outlet header 25 are employed. Also, a collector 29 a is employed near a liquid outlet 29.

In FIG. 15, the intermediate liquid outlet 28 is located in the outlet header 25 and the liquid outlet 29 is located in the inlet header 24, but in FIG. 16 the intermediate liquid outlet 28 and the liquid outlet 29 are located in the outlet header 25. Also, there are possible constructions when the intermediate liquid outlet 28 is located in the inlet header 24 and the liquid outlet 29 is located in the outlet header 25, and constructions when the intermediate liquid outlet 28 and the liquid outlet 29 are located in the inlet header 24.

Usually, the number of passes in the first condensation stage is larger than in the second condensation stage.

In the condenser units shown in FIGS. 12-15, the refrigerant channels extending between the inlet header 24 and outlet header 25, are oriented horizontally and the condensing refrigerant flow is routed from top to bottom. There is an option to use the condenser units shown in FIGS. 13-16 in a reverse direction, where the vapor inlet is at 29 instead of being at 27, the vapor outlet is at 27 instead of being at 29; and the intermediate liquid outlet 28 remains the same. In this case, the condensing refrigerant flow is routed from bottom to top.

Configurations as mentioned in U.S. Pat. No. 5,988,267 and in U.S. Pat. No. 5,762,566 are possible as well.

FIGS. 17-18 relate to shell-and-tube condenser units cooled, for instance, by water or brine. The shell-and-tube heat exchangers have one refrigerant pass, one pass for heated fluid, and three condensation stages.

The shell-and-tube condenser in FIG. 17 has an elongated cylindrical housing or shell 40. Inside the shell 40, there are a bundle of longitudinal heat transfer tubes 41. The shell 40 and the heat transfer tubes 41 are extended between a first tube sheet 42 and a second tube sheet 43. A first bonnet 44 is attached to the first tube sheet 42 at one end of the shell 40. A second bonnet 45 is attached to the second tube sheet 43 at the opposite end of the shell 40. The tube side is intended for a water stream. The first bonnet 44 has a water inlet 46 and the second bonnet 45 has a water outlet 47. The shell side is intended for a refrigerant stream. A refrigerant inlet 48, and three refrigerant outlets 49, 50, and 51, are arranged in the shell 40. Three vertical baffles 52 a, 52 b, and 52 c are installed inside the shell 40, providing three condensation zones. The refrigerant inlet 48 and the first refrigerant outlet 49 are located in the first condensation zone between the first tube sheet 42 and the first vertical baffle 52 a. The second refrigerant outlet 50 is located in the second condensation zone between the first vertical baffle 52 a and the third vertical baffle 52 c, and including the second vertical baffle 52 b. The third refrigerant outlet 51 is located in the third condensation zone between the third vertical baffle 52 c and the second tube sheet 43. The vertical baffles 52 a, 52 b, and 52 c direct refrigerant streams as shown by the arrows in FIG. 17.

The shell-and-tube condenser in FIG. 18 has longitudinal baffles 53 a and 53 b and vertical baffles 54 a and 54 b. A refrigerant inlet 48, and three refrigerant outlets 49, 50, and 51, are arranged in the shell 40. The refrigerant inlet 48 and the first refrigerant outlet 49 are located in the first condensation zone between the first vertical baffle 54 a and the second vertical baffle 54 b. The second refrigerant outlet 50 is located in the second condensation zone between a first end 42 and the first vertical baffle 54 a. The third refrigerant outlet 51 is located in the third condensation zone between the second vertical baffle 54 b and the second tube sheet 43. The longitudinal baffles 53 a and 53 b and the vertical baffles 54 a and 54 b direct refrigerant stream as shown by the arrows in FIG. 18.

In both FIG. 17 and FIG. 18, a first portion of refrigerant is condensed in the first condensation zone and the condensed portion is removed from the shell side through the first refrigerant outlet 49. A second portion of refrigerant is condensed in the second condensation zone and the condensed portion is removed from the shell side through the second refrigerant outlet 50. A third portion of refrigerant is condensed in the third condensation zone and the condensed portion is removed from the shell side through the third refrigerant outlet 51.

FIG. 19 combines a Rankine loop and a vapor compression loop. The Rankine loop includes a condenser unit 1, a receiver 55, a gravity-driven pumping unit 2 installed on a liquid line 5, a boiler unit 3, a high-pressure pipe 6, an expansion machine 4, and a low-pressure pipe 7. The vapor compression loop consists of a compressor 22, a discharge line 23, a portion of a low-pressure line 7 (which is a high-pressure line for the vapor compression loop), the condenser unit 1, the receiver 55, an expansion device 19, an evaporator unit 20, and a suction line 21. The compressor 22 and the expansion machine 4 may share a common shaft, and the energy obtained from the expansion process in the expansion machine 4 is used to drive, or assist in driving, the compressor 22. The compressor 22 and the expansion machine 4 may have a common casing, forming a hermetic unit. The Rankine loop generates mechanical energy and operates between a boiling (high) pressure and a condensing (low) pressure. The obtained mechanical energy is used to drive, or assist in driving, the compressor 22 operating between the condensing (high) and evaporating (low) pressures. The vapor compression loop driven by the compressor 22 provides cooling in the evaporator unit 20 and/or a heating in the condenser unit 1. The vapor compression loop may have a reversing valve to enable operation of the loop as a heat pump. Cooling and/or heating capacities generated by the vapor compression loop are regulated by the gravity-driven pumping unit 2.

While certain preferred embodiments of the present invention have been disclosed in detail, it is to be understood that various modifications in its structure may be adopted without departing from the spirit and scope of the invention as defined by the following claims. 

1. A Rankine system comprising: a closed-loop refrigerant cycle having an expansion machine, at least one condenser unit, at least one gravity-driven pumping unit, at least one boiler unit, and a control system; said boiler unit providing thermal contact and heat transfer interaction between a fluid carrying enthalpy of available thermal energy and a liquid refrigerant; said condenser unit providing thermal contact and heat transfer interaction between a fluid to be heated and a refrigerant vapor to be condensed; said gravity-driven pumping unit having an inlet valve, an outlet valve, and a staging zone between said inlet valve and said outlet valve; said inlet valve being connected to said condenser unit and; said outlet valve being connected to said boiler unit; one of said inlet valve and outlet valve being a normally open valve and the other being designed for bi-directional operation; said condenser unit being disposed at a higher elevation than that of said boiling unit; said inlet valve, said outlet valve, and the entire path between said condenser unit and said boiler unit being oriented downwardly to allow vapor refrigerant to freely move upward from said boiler unit to said condenser unit and to allow liquid refrigerant to freely drain by gravity downwards from said condenser unit to said boiler unit; said control system facilitating operation of said gravity-driven pumping unit by opening and closing said inlet valve and said outlet valve in a sequence which enables gravity-driven movement of liquid refrigerant from said condenser unit to said staging zone and then from said staging zone to said boiler unit, against a positive pressure differential between said boiler unit and said condenser unit.
 2. A Rankine system with as recited in claim 1 wherein said at least one gravity-driven pumping unit includes a liquid receiver positioned upstream of said inlet valve.
 3. A Rankine system as recited in claim 1 wherein said control system is programmed to open said inlet valve, allow for a time interval to fill said staging zone with liquid refrigerant, close said inlet valve, allow a time delay prior to opening said outlet valve, open said outlet valve, allow for a time interval to discharge refrigerant from said staging zone, close said outlet valve, allow for a time delay prior to opening said inlet valve, and repeat the above sequence.
 4. A Rankine system as recited in claim 3, wherein said control system is further programmed to assign nominal values to said time to fill the staging zone with refrigerant and to said time to discharge refrigerant from said staging zone, assign nominal values to said time delay prior to opening said outlet valve and to said time delay prior to opening said inlet valve, to provide maximal pumping capacity; assign values different from said nominal values of said time to fill staging zone with refrigerant and said time to discharge refrigerant from said staging zone, assign values larger than said nominal values of said time delay prior to opening said outlet valve and to said time delay prior to opening said inlet valve, to decrease pumping capacity.
 5. A Rankine system as recited in claim 1 wherein said control system is programmed to maintain superheat at an inlet to said expansion machine based on pressure and temperature monitored by a pressure sensor and a temperature sensor at said inlet to said expansion machine, and to accordingly decrease pumping capacity if superheat is decreased and increase pumping capacity if superheat is increased.
 6. A Rankine system as recited in claim 1 wherein said control system is programmed to open said inlet valve and said outlet valve, in order to relieve excessive pressure from a high pressure side of said Rankine system to a low-pressure side of said Rankine system, when at least one pressure sensor positioned within the system indicates excessive pressure elevation.
 7. A Rankine system as recited in claim 1 wherein said at least one gravity-driven pumping unit comprises a plurality of gravity driven pumps, with each gravity-driven pump housing an inlet valve, an outlet valve, and a staging zone.
 8. A Rankine system as recited in claim 7 wherein said control system is programmed to regulate pumping capacity by engaging a different number of said gravity-driven pumps.
 9. A Rankine system as recited in claim 1 wherein a number of boiling pressure levels is provided; said expansion machine has an inlet associated with a highest boiling pressure level, and a number of inlets introducing refrigerant streams into an expansion process associated with other boiling pressure levels; said boiler unit has a plurality of boilers connected in sequence, with respect to said fluid carrying enthalpy of available thermal energy; said gravity-driven pumping unit has a plurality of said gravity-driven pumping units; a number of said boiling pressure levels, a number of said boilers, total number of said inlets to said expansion machine, and a number of said gravity-driven pumps are the same; and each said gravity-driven pump feeds one boiler and one inlet to said expansion machine.
 10. A Rankine system as recited in claim 1 wherein a number of boiling pressure levels is provided; said expansion machine has a plurality of expansion machines, said boiler unit has the same plurality of boilers connected in sequence, with respect to said fluid carrying enthalpy of available thermal energy; said gravity-driven pumping unit has the same plurality of said gravity-driven pumping units; a number of said boiling pressure levels, a number of said boilers, a number of said expansion machines, and a number of said gravity-driven pumps are same; and each said gravity-driven pump feeds one boiler and one expansion machine.
 11. A Rankine system as recited in claim 10 wherein said plurality of expansion machines is connected in series.
 12. A Rankine system as recited in claim 10 wherein said plurality of said expansion machines is connected in parallel.
 13. A Rankine system as recited in claim 1 wherein said at least one condenser unit comprises a plurality of condensers connected in sequence, with respect to the fluid to be heated and with respect to refrigerant stream exiting said expansion machine; said gravity-driven pumping unit has the same plurality of said gravity-driven pumping units; each said condensers feeds one gravity-driven pumping unit with refrigerant liquid and feeds a next downstream condenser, if any, with refrigerant vapor.
 14. A Rankine system as recited in claim 1 wherein said outlet valve is installed to stop refrigerant flow in both directions and said inlet valve is installed to stop refrigerant flow in a direction from said boiler unit to said condenser unit.
 15. A Rankine system as recited in claim 14 wherein said inlet valve is a normally open flow control device and said outlet valve is a normally closed flow control device.
 16. A Rankine system as recited in claim 14 wherein said inlet valve is a normally closed flow control device and said outlet valve is a normally open flow control device.
 17. A Rankine system as recited in claim 14 wherein said outlet valve is a co-axial solenoid valve.
 18. A Rankine system as recited in claim 14 wherein said outlet valve is a motorized valve.
 19. A Rankine system as recited in claim 14 wherein said outlet valve is a modulation valve actuated by a stepper motor.
 20. A Rankine system as recited in claim 1 wherein said inlet valve is installed to stop refrigerant flow in both directions and said outlet valve is installed to stop refrigerant flow in a direction from said boiler unit to said condense unit.
 21. A Rankine system as recited in claim 20 wherein said inlet valve is a normally open flow control device and said outlet valve is a normally closed flow control device.
 22. A Rankine system as recited in claim 20 wherein said inlet valve is a normally closed flow control device and said outlet valve is a normally open flow control device.
 23. A Rankine system as recited in claim 20 wherein said inlet valve is a co-axial solenoid valve.
 24. A Rankine system as recited in claim 1 wherein said inlet valve and said outlet valve are installed to stop refrigerant flow in both directions.
 25. A Rankine system as recited in claim 24 wherein said inlet valve is a normally open flow control device and said outlet valve is a normally closed flow control device.
 26. A Rankine system as recited in claim 24 wherein said inlet valve is a normally closed flow control device and said outlet valve is a normally open flow control device.
 27. A Rankine system as recited in claim 24 wherein said inlet valve and said outlet valve are co-axial solenoid valves.
 28. A Rankine system as recited in claim 14 wherein said outlet valve is an assembly of two solenoid valves; a first solenoid valve exposed to said inlet valve and installed to stop refrigerant flow in a direction from said condenser unit to said boiler unit and; a second solenoid valve exposed to said boiler unit and installed to stop refrigerant flow in a direction from said boiler unit to said condenser unit.
 29. A Rankine system as recited in claim 28 wherein said inlet valve is a normally open flow control device and said second solenoid valve is a normally closed flow control device.
 30. A Rankine system as recited in claim 20 wherein said inlet valve is a normally closed flow control device and said second solenoid valve is a normally open flow control device.
 31. A Rankine system as recited in claim 14 wherein said outlet valve is an assembly of two solenoid valves; a first solenoid valve exposed to said inlet valve and installed to stop refrigerant flow in a direction from said boiler unit to said condenser unit and; a second solenoid valve exposed to said boiler unit and installed to stop refrigerant flow in a direction from said condenser unit to said boiler unit.
 32. A Rankine system as recited in claim 13 wherein said condenser unit has a two condensation stages having a vapor inlet, an inlet header, an outlet header, a plurality of refrigerant channels extending between said inlet header and said outlet header and sealed inside said inlet and outlet headers, an intermediate liquid outlet, a liquid outlet, means to route refrigerant flow from said vapor inlet to said intermediate liquid and liquid outlets, a first condensation stage associated with one portion of said refrigerant channels, a second condensation stage associated with another portion of said refrigerant channels, and means to remove a condensed liquid portion after said first condensation stage.
 33. A Rankine system as recited in claim 32 wherein said means to route refrigerant flow from said vapor inlet to said intermediate liquid and liquid outlets consist of at least one of a phase separator, a baffle, and a collector inside said inlet header and said outlet header.
 34. A Rankine system as recited in claim 32 wherein said means to remove condensed liquid portion after said first condensation stage consist of at least one of a phase separator, a baffle, and a collector inside said inlet header and said outlet header.
 35. A Rankine system as recited in claim 32 wherein said condensation stage has a plurality of coils, and a plurality of vapor inlets of said coils are connected to said vapor inlet of said condenser unit, a plurality of intermediate liquid outlets of said coils are connected to said intermediate liquid outlet of said condenser unit, and a plurality of liquid outlets of said coils are connected to said liquid outlet of said condenser unit.
 36. A Rankine system as recited in claim 32 wherein said two-stage condenser unit has a plurality of two-stage condenser coils.
 37. A Rankine system as recited in claim 1 and including a vapor compression system with a compressor wherein said expansion machine is connected at least to assist in driving said compressor and further wherein the capacity of said vapor compression system is regulated by the pumping capacity of said gravity-driven pumping unit.
 38. A Rankine system as recited in claim 18 wherein said staging zone has a relief valve connected to a point outside said gravity-driven pumping unit.
 39. A Rankine system as recited in claim 1 wherein said vapor compression system is a heat pump. 